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<FONT color="green">001</FONT>    /*<a name="line.1"></a>
<FONT color="green">002</FONT>     * Licensed to the Apache Software Foundation (ASF) under one or more<a name="line.2"></a>
<FONT color="green">003</FONT>     * contributor license agreements.  See the NOTICE file distributed with<a name="line.3"></a>
<FONT color="green">004</FONT>     * this work for additional information regarding copyright ownership.<a name="line.4"></a>
<FONT color="green">005</FONT>     * The ASF licenses this file to You under the Apache License, Version 2.0<a name="line.5"></a>
<FONT color="green">006</FONT>     * (the "License"); you may not use this file except in compliance with<a name="line.6"></a>
<FONT color="green">007</FONT>     * the License.  You may obtain a copy of the License at<a name="line.7"></a>
<FONT color="green">008</FONT>     *<a name="line.8"></a>
<FONT color="green">009</FONT>     *      http://www.apache.org/licenses/LICENSE-2.0<a name="line.9"></a>
<FONT color="green">010</FONT>     *<a name="line.10"></a>
<FONT color="green">011</FONT>     * Unless required by applicable law or agreed to in writing, software<a name="line.11"></a>
<FONT color="green">012</FONT>     * distributed under the License is distributed on an "AS IS" BASIS,<a name="line.12"></a>
<FONT color="green">013</FONT>     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.<a name="line.13"></a>
<FONT color="green">014</FONT>     * See the License for the specific language governing permissions and<a name="line.14"></a>
<FONT color="green">015</FONT>     * limitations under the License.<a name="line.15"></a>
<FONT color="green">016</FONT>     */<a name="line.16"></a>
<FONT color="green">017</FONT>    package org.apache.commons.lang3.concurrent;<a name="line.17"></a>
<FONT color="green">018</FONT>    <a name="line.18"></a>
<FONT color="green">019</FONT>    import java.util.concurrent.atomic.AtomicReference;<a name="line.19"></a>
<FONT color="green">020</FONT>    <a name="line.20"></a>
<FONT color="green">021</FONT>    /**<a name="line.21"></a>
<FONT color="green">022</FONT>     * &lt;p&gt;<a name="line.22"></a>
<FONT color="green">023</FONT>     * A specialized implementation of the {@code ConcurrentInitializer} interface<a name="line.23"></a>
<FONT color="green">024</FONT>     * based on an {@link AtomicReference} variable.<a name="line.24"></a>
<FONT color="green">025</FONT>     * &lt;/p&gt;<a name="line.25"></a>
<FONT color="green">026</FONT>     * &lt;p&gt;<a name="line.26"></a>
<FONT color="green">027</FONT>     * This class maintains a member field of type {@code AtomicReference}. It<a name="line.27"></a>
<FONT color="green">028</FONT>     * implements the following algorithm to create and initialize an object in its<a name="line.28"></a>
<FONT color="green">029</FONT>     * {@link #get()} method:<a name="line.29"></a>
<FONT color="green">030</FONT>     * &lt;ul&gt;<a name="line.30"></a>
<FONT color="green">031</FONT>     * &lt;li&gt;First it is checked whether the {@code AtomicReference} variable contains<a name="line.31"></a>
<FONT color="green">032</FONT>     * already a value. If this is the case, the value is directly returned.&lt;/li&gt;<a name="line.32"></a>
<FONT color="green">033</FONT>     * &lt;li&gt;Otherwise the {@link #initialize()} method is called. This method must be<a name="line.33"></a>
<FONT color="green">034</FONT>     * defined in concrete subclasses to actually create the managed object.&lt;/li&gt;<a name="line.34"></a>
<FONT color="green">035</FONT>     * &lt;li&gt;After the object was created by {@link #initialize()} it is checked<a name="line.35"></a>
<FONT color="green">036</FONT>     * whether the {@code AtomicReference} variable is still undefined. This has to<a name="line.36"></a>
<FONT color="green">037</FONT>     * be done because in the meantime another thread may have initialized the<a name="line.37"></a>
<FONT color="green">038</FONT>     * object. If the reference is still empty, the newly created object is stored<a name="line.38"></a>
<FONT color="green">039</FONT>     * in it and returned by this method.&lt;/li&gt;<a name="line.39"></a>
<FONT color="green">040</FONT>     * &lt;li&gt;Otherwise the value stored in the {@code AtomicReference} is returned.&lt;/li&gt;<a name="line.40"></a>
<FONT color="green">041</FONT>     * &lt;/ul&gt;<a name="line.41"></a>
<FONT color="green">042</FONT>     * &lt;/p&gt;<a name="line.42"></a>
<FONT color="green">043</FONT>     * &lt;p&gt;<a name="line.43"></a>
<FONT color="green">044</FONT>     * Because atomic variables are used this class does not need any<a name="line.44"></a>
<FONT color="green">045</FONT>     * synchronization. So there is no danger of deadlock, and access to the managed<a name="line.45"></a>
<FONT color="green">046</FONT>     * object is efficient. However, if multiple threads access the {@code<a name="line.46"></a>
<FONT color="green">047</FONT>     * AtomicInitializer} object before it has been initialized almost at the same<a name="line.47"></a>
<FONT color="green">048</FONT>     * time, it can happen that {@link #initialize()} is called multiple times. The<a name="line.48"></a>
<FONT color="green">049</FONT>     * algorithm outlined above guarantees that {@link #get()} always returns the<a name="line.49"></a>
<FONT color="green">050</FONT>     * same object though.<a name="line.50"></a>
<FONT color="green">051</FONT>     * &lt;/p&gt;<a name="line.51"></a>
<FONT color="green">052</FONT>     * &lt;p&gt;<a name="line.52"></a>
<FONT color="green">053</FONT>     * Compared with the {@link LazyInitializer} class, this class can be more<a name="line.53"></a>
<FONT color="green">054</FONT>     * efficient because it does not need synchronization. The drawback is that the<a name="line.54"></a>
<FONT color="green">055</FONT>     * {@link #initialize()} method can be called multiple times which may be<a name="line.55"></a>
<FONT color="green">056</FONT>     * problematic if the creation of the managed object is expensive. As a rule of<a name="line.56"></a>
<FONT color="green">057</FONT>     * thumb this initializer implementation is preferable if there are not too many<a name="line.57"></a>
<FONT color="green">058</FONT>     * threads involved and the probability that multiple threads access an<a name="line.58"></a>
<FONT color="green">059</FONT>     * uninitialized object is small. If there is high parallelism,<a name="line.59"></a>
<FONT color="green">060</FONT>     * {@link LazyInitializer} is more appropriate.<a name="line.60"></a>
<FONT color="green">061</FONT>     * &lt;/p&gt;<a name="line.61"></a>
<FONT color="green">062</FONT>     *<a name="line.62"></a>
<FONT color="green">063</FONT>     * @since 3.0<a name="line.63"></a>
<FONT color="green">064</FONT>     * @version $Id: AtomicInitializer.java 1088899 2011-04-05 05:31:27Z bayard $<a name="line.64"></a>
<FONT color="green">065</FONT>     * @param &lt;T&gt; the type of the object managed by this initializer class<a name="line.65"></a>
<FONT color="green">066</FONT>     */<a name="line.66"></a>
<FONT color="green">067</FONT>    public abstract class AtomicInitializer&lt;T&gt; implements ConcurrentInitializer&lt;T&gt; {<a name="line.67"></a>
<FONT color="green">068</FONT>        /** Holds the reference to the managed object. */<a name="line.68"></a>
<FONT color="green">069</FONT>        private final AtomicReference&lt;T&gt; reference = new AtomicReference&lt;T&gt;();<a name="line.69"></a>
<FONT color="green">070</FONT>    <a name="line.70"></a>
<FONT color="green">071</FONT>        /**<a name="line.71"></a>
<FONT color="green">072</FONT>         * Returns the object managed by this initializer. The object is created if<a name="line.72"></a>
<FONT color="green">073</FONT>         * it is not available yet and stored internally. This method always returns<a name="line.73"></a>
<FONT color="green">074</FONT>         * the same object.<a name="line.74"></a>
<FONT color="green">075</FONT>         *<a name="line.75"></a>
<FONT color="green">076</FONT>         * @return the object created by this {@code AtomicInitializer}<a name="line.76"></a>
<FONT color="green">077</FONT>         * @throws ConcurrentException if an error occurred during initialization of<a name="line.77"></a>
<FONT color="green">078</FONT>         * the object<a name="line.78"></a>
<FONT color="green">079</FONT>         */<a name="line.79"></a>
<FONT color="green">080</FONT>        public T get() throws ConcurrentException {<a name="line.80"></a>
<FONT color="green">081</FONT>            T result = reference.get();<a name="line.81"></a>
<FONT color="green">082</FONT>    <a name="line.82"></a>
<FONT color="green">083</FONT>            if (result == null) {<a name="line.83"></a>
<FONT color="green">084</FONT>                result = initialize();<a name="line.84"></a>
<FONT color="green">085</FONT>                if (!reference.compareAndSet(null, result)) {<a name="line.85"></a>
<FONT color="green">086</FONT>                    // another thread has initialized the reference<a name="line.86"></a>
<FONT color="green">087</FONT>                    result = reference.get();<a name="line.87"></a>
<FONT color="green">088</FONT>                }<a name="line.88"></a>
<FONT color="green">089</FONT>            }<a name="line.89"></a>
<FONT color="green">090</FONT>    <a name="line.90"></a>
<FONT color="green">091</FONT>            return result;<a name="line.91"></a>
<FONT color="green">092</FONT>        }<a name="line.92"></a>
<FONT color="green">093</FONT>    <a name="line.93"></a>
<FONT color="green">094</FONT>        /**<a name="line.94"></a>
<FONT color="green">095</FONT>         * Creates and initializes the object managed by this {@code<a name="line.95"></a>
<FONT color="green">096</FONT>         * AtomicInitializer}. This method is called by {@link #get()} when the<a name="line.96"></a>
<FONT color="green">097</FONT>         * managed object is not available yet. An implementation can focus on the<a name="line.97"></a>
<FONT color="green">098</FONT>         * creation of the object. No synchronization is needed, as this is already<a name="line.98"></a>
<FONT color="green">099</FONT>         * handled by {@code get()}. As stated by the class comment, it is possible<a name="line.99"></a>
<FONT color="green">100</FONT>         * that this method is called multiple times.<a name="line.100"></a>
<FONT color="green">101</FONT>         *<a name="line.101"></a>
<FONT color="green">102</FONT>         * @return the managed data object<a name="line.102"></a>
<FONT color="green">103</FONT>         * @throws ConcurrentException if an error occurs during object creation<a name="line.103"></a>
<FONT color="green">104</FONT>         */<a name="line.104"></a>
<FONT color="green">105</FONT>        protected abstract T initialize() throws ConcurrentException;<a name="line.105"></a>
<FONT color="green">106</FONT>    }<a name="line.106"></a>




























































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